Although 5-fluorouracil (5-FU) or its derivatives are clinically useful antitumor agents for the treatment of various solid tumors, in general they are still not satisfactory in terms of efficacy and safety. These drawbacks are mainly due to rapid inactivation of 5-FU by dihydropyrimidine dehydrogenase (DPD) and/or the unsatisfactory delivery of 5-FU to tumor tissues with respect to tumor selectivity. Attempts to enhance the antitumor activity of 5-FU or its derivatives by inhibition of DPD have already been reported: the co-administration of 5-FU or its derivative with a DPD inhibitor such as uracil [U.S. Pat. No. 4,328,229], 5-ethynyluracil [WO92/04901], 5-chloro-2,4-dihydroxypyridine [U.S. Pat. No. 5,525,603] etc. Such co-administration resulted in enhancement of the antitumor activity of 5-FU or its derivatives, but the safety profile was not so improved due to insufficient selectivity in delivering the DPD inhibitor to tumor tissues (as a consequence, 5-FU level is increased in both tumor and plasma).